Big controller little motor?

rjo__

Regarding the World's cheapest robotic telepresence.

So, the rules were that I could use any kind of controller... costs excluded. I chose a Propeller quickstart board for robot logic and a Sabertooth2X60 motor controller.http://www.dimensionengineering.com/products/sabertooth2x60 The motors are Faulhaber look alikes. http://www.goldmine-elec-products.com/prodinfo.asp?number=G18335

I know... this sounds incredibly stupid...using a 60A (6-30V) controller for a half amp, 6-9V motor. . (I had been using Ray's Merlin, hooked up over bluetooth, and everything was dandy. But I wanted to test my Sabertooth board before hooking it up to my semi-autonomous wheelchair(named "Evntuly")... so there is some logic here. I was using a fully charged 2000mAh RC battery pack from Radioshack

The first thing I found out is that when Dimension Engineering says TTL, what they mean is a full 5V for logic 1. That's a problem. Can't use 3V serial...can't use direct PWM from a Propeller unless you only want to go in one direction. And if you have the sabertooth set for PWM and you don't give it a signal... it goes full speed backwards. 2.5V is neutral. So, without 2.5V, you are going somewhere...fast, like it or not.
Not the problem I speak of here, just a few wrinkles I could live without.

So, I got out my $4 Parallax analog joystick and hooked it up. At 2.5 Volts, my little motors didn't move just as they shouldn't... but they did get hot and they appear to be in a braking mode... with lots of torque. Everything else was pretty much as expected... motors move when they should.

Is heating and braking to be expected? Can I expect the same when I hook up my 24V 15Amp motors?

I would show you pictures, except the wheels of my little Zippy are off again. This time the wobble in one of my peanut butter jar lid (wheels) was causing too much friction. So, I've decided to change the design.

Thanks.

Rich

Lawson

By its self, I wouldn't expect using an over sized controller to cause problems. What supply voltage are you using with the small Faulhaber motors? I suspect the heating is a combination of two factors. First the controller likely uses "locked anti-phase" pwm near zero speed. This switches the motor between forward and reverse instead of forward and off or reverse and off. The big advantage is that there is no dead-zone at zero speed. The big disadvantage is more motor heating. Second, that little motor of yours likely has an ironless rotor. Motor's with an ironless rotor have extra low inductance and need higher PWM speeds or series inductors to avoid heating at idle.

they appear to be in a braking mode... with lots of torque.

That is normal operation for a controller with synchronous rectification. The synchronous rectification lets power flow from the motor back to the battery when needed. The net effect is that the motor resists speeding up as strongly as it resists slowing down. (very useful most of the time)

Lawson

LoopyByteloose

If you need full 5V TTL, there are simple logic chips that output a full +5. The 74LS06 is a hex inverted. But two inverters in series output a normal signal and you have 6 inverters in one package. The 74LS07 doesn't invert, so you would have 6 +5 outputs. These are suppose to accept 3.3v logic and work well. If there are problems, a 10K pullup to 3.3v on the input side would likely resolve things.

Not sure if you motor control is responding to the logic poorly, or if you are not providing a high enough voltage to overcome the h-bridges internal voltage drops. You might just try operating the h-bridge from a regulated +5V directly to confirm you are getting the motor performance you want. While the motor is turning, you can measure the actual voltage to the motor. If it is too low, you may have to adjust your power source to a higher voltage.

If you do decide to use the 74ls07, they are open collector outputs. That means you have to provide 10K pullups +5 on the outputs.